1. Technical Field of the Invention
The present invention relates to an optical beam scanning apparatus and an image forming apparatus equipped with the optical beam scanning apparatus, and more particularly, to an optical beam scanning apparatus configured to be capable of performing rotary adjustment of the light source about the optical axis and an image forming apparatus equipped with the optical beam scanning apparatus.
2. Related Art
An image forming apparatus of the electrophotographic method, such as a laser printer, a digital copying machine, and a laser facsimile machine, is equipped with an optical beam scanning apparatus that forms an electrostatic latent image on the photoconductive drum by irradiating a laser beam (optical beam) on the surface of the photoconductive drum and scanning the laser beam thereon.
Recently, in order to increase the scanning rate on the surface of the photoconductive drum, there has been proposed a method (multi-beam method) for increasing the number of laser beams scanned at a time by providing plural light sources (laser diodes) to a single laser unit. According to the multi-beam method, plural beams for respective color components (for example, yellow, magenta, cyan, and black) emitted from the corresponding light sources undergo processing in the pre-deflection optical systems, while they are combined into a single beam to go incident on the polygon mirror. The beam deflected on the polygon mirror passes through the fθ lens forming the post-deflection optical system, after which it is separated into beams of the respective color components that are irradiated onto the photoconductive drums of the respective color components.
Incidentally, it is necessary for an optical beam scanning apparatus and an image forming apparatus using plural light sources (laser diodes) to perform rotary adjustment of the light sources (laser diodes) about the optical axis to maintain a specific sub-scanning beam pitch on the photoconductive drums. To be more specific, for example, in the case of 600 dpi (Dot Per Inch), it is necessary to perform rotary adjustment of the light source (laser diode) about the optical axis to maintain 42 μm as the sub-scanning beam pitch, and for example, in the case of 1200 dpi, it is necessary to perform rotary adjustment of the light source (laser diode) about the optical axis to maintain 21 μm as the sub-scanning beam pitch. Further, it is also necessary for the light source (laser diode) to match the optical axes with the collimator lens.
Such being the case, there have been proposed various techniques for the rotary adjustment about the optical axis and the optical axis matching with the collimator lens for an optical beam scanning apparatus and an image forming apparatus.
According to the technique proposed in JP-A-2003-161904, when an optical housing provided with laser beam sources, deflection means, and an image forming optical system is attached along guiding means, the position of the optical housing in the scanning direction is determined by scanning direction positioning means, while the position of the optical housing in the optical axis direction is determined by pressing the optical housing against optical axis direction positioning means. This configuration makes it possible to perform assembly and adjustment works of the optical housing with respect to the image forming apparatus precisely with ease.
Also, according to the technique proposed in JP-A-2003-98461, a lens holder holding a cylindrical cell enclosing a lens made of resin is layered on a lens holder holding a cell enclosing respective lenses made of glass, while each lens holder is set to be able to adjust the position in the optical axis direction. While an adjustment is being made, movements of a lever that oscillates with rotations of the screw are scaled down and transmitted to the holders via a linking shaft. This configuration makes it possible to adjust and hold the cylindrical cell within a long groove in a perpendicular portion of the holder by a hold cam in a direction perpendicular to the optical axis and in the direction of rotation about the optical axis.
Further, according to the technique proposed in JP-A-2004-246032, a first optical system comprises a cylinder lens and a lens holding member holding the cylinder lens, and the lens holding member is attached to the housing of the optical beam scanning apparatus to be able to move along the optical axis direction. This configuration makes it possible to hold the lens and adjust the position thereof in the optical axis direction using a simple structure.
Generally, by taking a tolerance of components into account, it is preferable to perform rotary adjustment about the optical axis in the optical beam scanning apparatus and the image forming apparatus in a state where all the unit components of the optical beam scanning apparatus and the image forming apparatus have been assembled.
However, when the rotary adjustment about the optical axis is performed in an assembled state, the rotary adjustment about the optical axis is normally performed by making access to the light source (laser diode) from behind because of supporting and screwing for the rotary adjustment. This requires a space to allow access to the light source (laser diode) from behind for performing adjustment, and therefore poses a problem that the units of the optical beam scanning apparatus and the image forming apparatus are increased in size.
To be more concrete, as is shown in FIG. 1, roughly speaking, a pre-deflection optical system 1-b and a post-deflection optical system 1-c are provided within a unit 1-a of the optical beam scanning apparatus. Laser units 1-d through 1-g, for example, of respective colors are disposed in the pre-deflection optical system 1-b. However, when rotary adjustment about the optical axis is performed by making access to the light sources (laser diodes 1-d through 1-g) from behind, a space to allow access in the directions indicated by arrows for performing adjustment is necessary. The unit 1-a of the optical beam scanning apparatus therefore has to be increased as large as the unit 1-h. 
As a countermeasure, a hole may be provided in the unit of the optical beam scanning apparatus, so that access is made to the light sources (laser diodes) from the outside of the unit. This countermeasure, however, requires a die of the sliding structure for the unit, which deteriorates the accuracy or increases the cost.
These problems cannot be solved by the techniques proposed in JP-A-2003-161904, JP-A-2003-98461, and JP-A-2004-246032.